Recreational Fishery Stock Assessment 1999 Final Report

PHOTO 1. Clear Lake Aerial Photo. June 2001. Click image to view 72K JPG

Introduction

This report presents the results of a stock assessment of Clear Lake, completed by Ted Zimmerman and Sean Barry on June 25, 1998. Clear Lake was initially stocked with rainbow trout in 1984. The assessment was carried out as part of a region-wide stocked lake monitoring program, in which Clear Lake ranked as a high priority for investigation. This report follows up on previous surveys conducted by the Ministry in 1985 and 1988, and provides management recommendations for the Clear Lake recreational fishery.

Methods

An experimental, 91.2m sinking monofilament gill net was set in Clear Lake on June 25,1998 according to the methods outlined in the Resource Inventory Committee document Fish Collection Methods and Standards. The net was deployed at 11:32 and retrieved the same day at 17:28, for a total soak time of 5 hours, 56 minutes.

FIGURE 1. 1998 Net set location Click image to view 45K GIF

The net was set 8 m from the east shore in an E-W direction (Figure 1). The net ranged in depth from the surface to 14m.

All trout collected were sampled for fork length and weight. Due to the large numbers of yearlings captured, sex and maturity were measured from only a subsample of the smaller fish (Appendix 1). Scale samples were collected from all fish larger than 180mm, and from a sub-sample of those smaller than 180mm. The structures were sent to Darlene Gillespie of TimeMark Consulting Ltd.(Nanaimo, B.C.) for ageing. Upon return to the Ministry, scale prints were subsequently checked and ages were adjusted by the author to account for anomalies between the ages reported by the contractor and the known stock schedule and age composition of rainbow trout in Clear Lake.

Results and Discussion

CATCH SUMMARY

The net catch yielded 83 rainbow trout (RB), for a catch per unit effort of 14.0 RB per net-hour. No other fish were netted. See Appendix 1 for individual rainbow trout sample data.

ANALYSIS

FIGURE 2. Length vs. weight of rainbow trout captured in Clear Lake, June 1998.

Condition

The weight of Clear Lake rainbow trout increased according the equation W = 1E-5 x L^2.9908; this relationship is expressed in Figure 2. This figure shows that Clear Lake rainbow trout are generally in good condition, however the 5-year-old cohorts were slightly lighter for their length than the younger cohorts sampled. This trend has been observed across numerous lakes in the region, and is a normal occurrence for larger planktivorous trout. The older fish also appeared to be quite healthy with no noticeable parasites or lesions.

FIGURE 3. Length frequency distribution of rainbow trout captured in Clear Lake, comparing 1998 and 1992 results.

Size Distribution

Figure 3 compares the 1988 and 1998 length-frequency distributions of rainbow trout sampled in Clear Lake. The 1998 sample is dominated by large yearlings, which were stocked into Clear Lake on May 28. The 1993 and 1995 cohorts are represented by the 310-350 mm and 390-420 classes respectively. By contrast, the sample collected in October 1988 contains no fish larger than 290 mm, even though the sample was obtained after a full growing season and is represented by age classes 1 to 4. This suggests that the growth of each cohort was constrained such that their size distributions overlapped between year classes.

Growth

It can be seen from Figure 4 that the 1988 population exhibited slower growth relative to those fish sampled in 1998. No fish were older than 4 years in the 1988 sample, however several 5 year old fish were obtained in 1998. The mean length of an age 3 rainbow trout increased from 244mm in 1988 to 321mm in 1998, an increase of 32%. This increase in growth is also reflected in the change to the mean weight of age 3 fish (not shown), from an average of 154g to 395g, an increase of 156%. Had the 1998 survey been conducted in October, the difference in growth rate would have been even more pronounced.

FIGURE 4. A comparison of 1988 vs. 1998 Clear Lake rainbow trout growth rates.

Several factors are likely responsible for the observed increase in growth. The stocking frequency was changed from yearly to alternate years in 1988, the stocking rate was reduced from 2,500 fish to 1,500 fish in 1996, and the mean size at stocking increased during each stocking event to a maximum of 17.2g in 1996. A growth response may be attributable to changes in the strain of fish stocked as well, although this is difficult to quantify as there is no data available on relative growth rates of the various strains currently being stocked.

While the data suggests that growth has indeed improved in recent years, it should be noted that some level of error was associated with the scale reading process for the current study. Eight fish were originally classified as 4-year-olds, yet it is assumed that there is no natural recruitment in Clear Lake (since the lake has no inlets, required for spawning) and therefore no fish of this year class would be expected in the sample. A secondary examination of the scale prints revealed that with the exception of two scales, all the fish classified as 4-year-olds were likely either 3 or 5 year old fish. The two exceptions (sample no. 25 and 28) were difficult to classify and were subsequently assigned as 3-year-olds. The original and adjusted ages of each fish can be found in Appendix 1.

Management Recommendations

The rainbow trout fishery in Clear Lake has shown a remarkable improvement since 1988, a change that is no doubt attributable to the reductions in both the number of fish released and the frequency of those releases. The initial stocking rate likely exceeded the productive capacity of the lake, resulting in excessive intraspecific competition and abated growth amongst stocked fish. Currently, Clear Lake contains rainbow trout which are well conditioned and reasonably sized, and should provide a satisfactory recreational fishing experience. No further changes are recommended to the management strategy for this lake.

Literature Cited

Westcott, R. 1988. Note to file regarding the 1988 stock assessment of Clear Lake. Ministry of Environment, Lands and Parks. Prince George.

For More Information:

Contact :Ted Zimmerman
Sr. Fisheries Biologist, Omineca sub-Region
Prince George, B.C.
250-565-6852